1. Technical Field
The present invention relates generally to vertical cavity lasers, and more specifically to monolithic arrays of vertical cavity lasers.
2. Background Art
Vertical cavity surface emitting lasers (VCSELS) are semiconductor devices, which are revolutionizing the field of telecommunications. They generally consist of a pair of semiconductor mirrors defining a resonant cavity containing a gain medium of semiconductor materials for amplifying light.
VCSELs have relatively high efficiency, small size, low weight, low power consumption, and the capability to be driven by low-voltage power. They can operate in a single longitudinal mode, or frequency, and produce a circular beam of laser light that can easily be coupled into optical fibers. The surface emission feature allows devices to be packed densely on a wafer, so that two-dimensional arrays are fabricated relatively easily.
VCSELS use semiconductor materials comprised of elements such as aluminum, indium, gallium, arsenic, nitrogen, and phosphorous as the gain medium, and alternating high and low index of refraction materials such as silicon and silicon dioxide for the semiconductor mirrors or distributed Bragg reflectors (DBRs).
The lasing wavelength of a VCSEL is determined by the optical height of its resonant cavity. Most commonly the optical height, and thus the wavelength, is determined by the thicknesses of the semiconductor layers in the devices. These thicknesses are set during the growth of the semiconductor layers and are nominally the same for all the VCSELs on given wafer.
The resonant cavity of some VCSELs also includes an air gap, where the size of the air gap partly determines the lasing wavelength.
A monolithic multiple-wavelength VCSEL array requires side-by-side fabrication of VCSELs on a wafer where the VCSELs need to be exactly the same except with controlled, different lasing wavelengths. This presents a problem because the processing used on the wafer must assure that the threshold gain at which lasing begins, the current usage, the efficiency, the losses of light in the resonant cavity, the amplification of the gain material, and the light transmission of the DBR all remain the same. At the same time, the same processing must produce different lasing wavelengths, which is most commonly realized by changing the optical height of the resonant cavity.
One prior art method to making a monolithic multiple wavelength VCSEL array is non-uniform growth due to thermal gradient. The backside of a substrate is patterned prior to epitaxial growth in a molecular beam epitaxy reactor. The resulting backside pattern produces a thermal gradient on the surface of the substrate when the wafer is heated. Because growth rate is temperature dependent, there is a variable material thickness and hence a variable laser wavelength along the thermal gradient. One disadvantage of this approach is the fact that the arrays are limited to linear geometries. To date, there have been problems controlling the wavelengths precisely and repeatedly over large areas of the wafer.
An alternate prior art method is laterally confining each laser prior to epitaxial growth by either etching a mesa or patterning windows in an oxide mask. This process is known as xe2x80x9cselective area growthxe2x80x9d. Growth rate and composition are functions of the lateral dimension. The method is problematic because it is sensitive to growth conditions and may vary from reactor to reactor or from growth to growth. In both of the aforementioned prior art methods, the proximity of different wavelength devices in an array is limited.
Another prior art method is to grow a partial VCSEL structure including the lower DBR, the gain material, and some part of the upper DBR. The wafer is masked and anodically oxidized to some controlled oxide thickness over the exposed portions. A selective etch is then used to remove the oxide. This process is repeated to create different effective resonant cavity lengths for each laser in an array. The remainder of the VCSEL structure is regrown over the patterned wafer. Besides requiring a large number of process steps, each etch is sensitive to voltage and concentration variations that cause problems, which affect the depth, resulting in reduced control over wavelength spacing between devices.
Another prior art method is to grow a partial VCSEL structure including the lower DBR, the gain material, and a series of etch-stop layers that can be selectively etched away. The wafer is repeatedly masked and etched so that different amounts of material are removed from the different VCSELs in the array. The wafer is then re-introduced into the semiconductor growth apparatus and the deposition of the remaining layers is performed. This approach requires multiple masking and etching steps to achieve the different etch depths. This becomes unmanageable when the number of VCSELs is greater than a few. In addition, this process requires the upper portion of the semiconductor material must be epitaxially re-grown after the etching process is complete. This increases both the complexity and the cost of the fabrication process.
Despite the large number of methods developed, a method that provides VCSELs having the same accuracy as planar epitaxial growth and not requiring a large number of masking steps or multiple epitaxial growths has long been sought but has long eluded those skilled in the art.
The present invention provides a monolithic array of vertical cavity lasers with different emission wavelengths on a single wafer, and method of manufacture therefor. A first reflector is over the semiconductor substrate with a photoactive semiconductor layer. A reflector support defines first and second air gaps with the photoactive semiconductor layer. The second and third air gaps are made to be different from each other by geometric differences in the reflector support structure. Second and third reflectors are formed over the reflector support whereby a first laser is formed by the first reflector, the photoactive semiconductor structure, the first air gap, and the second reflector and whereby a second laser is formed by the first reflector, the photoactive semiconductor structure, the second air gap, and the third reflector. The emission wavelengths of the first and second lasers are different because of the different sizes of the first and second air gaps. Only one mask is required to set the air gaps for an array containing an arbitrary number of lasers.
The above and additional advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description when taken in conjunction with the accompanying drawings.